This invention relates to a method of detecting and monitoring corrosion on both onshore and offshore pipelines especially but not exclusively by utilising low frequency signal propagation communication systems.
Pipelines used for the transport of oil both on and offshore are subject to corrosion effects both from the material being transported and from the external environment. In order to ensure that the mechanical integrity of the pipeline is not compromised there is a need to monitor any corrosion that takes place. In certain circumstances, visual inspection of the pipeline is possible. However internal inspection and inspection of offshore pipelines in this way is generally not practical.
It is known that corrosion effects in a metallic pipeline will have an effect on electrical signals that are transmitted along a section of pipeline. Various proposals have previously been made to make use of electrical effects to monitor and measure corrosion rates in metal structures. Examples of methods for monitoring and measuring corrosion in metal structures using electrical techniques are described in U.S. Pat. Nos. 4,575,678, 4,658,365, 5,139,627 and 5,370,776. In each of these there is either a dedicated electrical connection made to the structure using parts of the structure to form electrodes or special electrodes attached to the structure. Various techniques are used for applying and analyzing electrical signals to obtain some measure of corrosion effects.
All of these techniques however share the disadvantage and limitation that they rely on the addition of extra components and specific dedicated external circuitry to the pipeline or structure in order to apply and measure electrical signals. In certain circumstances, for example offshore, this may be difficult to achieve.
It is an object of the present invention to provide a means of analyzing the electrical effects caused by corrosion in metal structures without the addition of extra external connections and circuitry.
According to the present invention there is provided a method of corrosion monitoring of pipelines comprising separating out a noise signal from a reference signal transmitted on a pipeline and analyzing said noise signal to provide an indication of the presence of corrosion effects.
Preferably the reference signal comprises a data communications signal.
The reference signal may be a DC signal or a low frequency AC signal. Most preferably, the reference signal has a signal frequency of around 100 Hz.
Preferibly also, the noise signal is separated out into elements representative of specific corrosion effects on the pipeline.
More preferably, the separate noise elements are analysed by comparison to noise signals representative of a range of corrosion effects to identify the presence and extent of any of said corrosion effects.
Most preferably, the separate noise elements are additionally analysed to determine the positional source of each noise element and so identify the location on the pipeline of each corrosion effect.
This may, for example, be by transmitting signals in both directions and analyzing their interaction.
The corrosion effects being monitored may for example include general corrosion, localised corrosion, sacrificial anode cathodic protection, CO2 corrosion and seawater corrosion.